The present invention relates to generating images using a coincidence event detector. More particularly, the present invention relates to normalizing the triggering events in a coincidence event detector, thus reducing image artifacts in the generated image.
Many types of equipment are designed to detect physical events such as particle-matter interactions. In the prior art, event detection has been widely used in scientific research and medicine to generate images of a patient""s internal structures. For example, nuclear medicine cameras (also referred to as a gamma cameras) may perform positron emission tomography (xe2x80x9cPETxe2x80x9d) on emitted photon pairs to map the originating location and energy of the emitted photon pairs, thus allowing the nuclear medicine cameras to generate images of tumors or other cancerous developments.
Typically, to generate the image, a patient is injected with a radio pharmaceutical (such as Flouro Deoxi Glucose) that interacts with electrons in the body of the patient. As a result of the interaction, positrons are annihilated and gamma rays, including photon pairs, are emitted from the body of the patient in directions of travel that are 180xc2x0 apart from each other. An example of a prior art PET may be found in the dual single-photon completed emission tomography (xe2x80x9cSPECTxe2x80x9d)/PET system of U.S. Pat. No. 5,608,221 of Bertelsen et. al (issued Mar. 4, 1997 and assigned to ADAC Laboratories of Milpitas, Calif.).
Certain nuclear medicine camera systems include two planar detector assemblies that are positioned on opposite sides of the patient. Each detector assembly includes a crystal covered by an array of photo multiplier tubes (xe2x80x9cPMTsxe2x80x9d). To perform the PET, nuclear medicine cameras detect the collision of photons with the detector assemblies. Specifically, when a photon comes in contact with the crystal of a detector assembly, a scintillation occurs. In response to the scintillation, the PMTs exposed to the scintillation event generate electrical outputs. Accordingly, the electrical output of the PMTs may be used to detect the emission of a coincidence photon pairxe2x80x94hereinafter referred to as a coincidence event.
As previously described, certain nuclear medicine camera systems include two planar detector assemblies. Accordingly, to detect a coincidence event nuclear medicine cameras use a PET coincidence detection system. In particular, in a PET coincidence detection system, the PMTs of a first detector assembly are divided into spatial quadrants (or zones) and the electrical outputs of the zones are summed. The summated outputs are digitized and logically combined with the digitized outputs of all the zones in the first detector assembly, thus generating a first coincidence trigger. Similarly, the PMT outputs of the second detector are summed and digitized to generate a second coincidence trigger. Accordingly, if the two coincidence triggers transition within a predetermined time window (typically 10-20 nanoseconds), the scintillations causing the transition of the coincidence triggers are deemed to be from a coincidence event. Thus, the location and energy of the scintillations on the PMTs may be used to 5 determine the origin of the event.
FIG. 1 is a block diagram of a prior PET coincidence detection system 100. System 100 includes two detectors 166 and 168. Detector 166 is divided into four zones 166 (1), 166 (2), 166 (3), and 166 (4). Each of zones 166 (1-4) include multiple PMTs. Zone 166 (1) operates similarly to other zones in detector 166 and corresponding zones in detector 168. Accordingly, zone 166 (1) will be described as an example of how a zone of a detector operates. When any PMT in zone 166 (1) detects a scintillation event resulting from a collision of a photon with the crystal of detector 166 (not shown), an analog signal is sent to summing circuit 101. Summing circuit 101 receives signals from all of the PMTs in zone 166 (1) and sums their amplitudes in a known manner for example via a series weighted resistors. Summing circuit 101 outputs a signal to constant fraction discriminator (xe2x80x9cCFDxe2x80x9d) 131. CFD 131 operates as a trigger detector in an amplitude independent manner. CFD 131 outputs a zone trigger signal to programmable delay 141.
Programmable delay 141 is typically controlled by a processor of system 100 and is used to vary the delay of the trigger signal output by CFD 131 during calibration of system 100. Zones 166 (2), 166 (3), and 166 (4) operate in the same manner as zone 166(1), each outputting a signal from their respective programmable delay circuits indicating that a coincidence event has been detected. The outputs of programmable delays 141, 142, 143, and 144 are input to OR gate 156. Detector trigger signal 170 is active on the output of OR gate 156 when any event is detected in a zone of detector 166. Detector trigger signal 170 is input to common delay CDA 160, which is associated with a detector 166. CDA 160 is a programmable delay circuit that is used to vary the delay of detector trigger signal 170 with respect to detector trigger signal 171. Adjusted detector trigger signals 174 and 176 are input to coincidence detection circuit 164. Coincidence detection circuit 164 typically performs an operation such as an AND operation for determining when scintillation events have been detected simultaneously in detectors 166 and 168.
As illustrated in FIG. 1, system 100 includes multiple delay elements to synchronize the detection of coincidence events. In particular, programmable delays 141-144 and 151-154 are adjusted to compensate for variances such as response time of different PMTs, different cable lengths between zones, and different component tolerances between zones. Additionally, CDA 160 and CDB 161 are adjusted for the purpose of synchronizing the global, or common, delays of signals 170 and 171 due to cable lengths between OR gates 156 and 158 and coincidence detection circuit 164.
Although a PET system using system 100 may generate accurate images based on the detection of events, the coincidence detection system has numerous disadvantages. One disadvantage, results from the non-uniformity of the images generated by the PET. In particular, the low number of zones in system 100 create xe2x80x9cdead areasxe2x80x9d during the detection of multiple events. For example, if an event causes a scintillation in the PMTs shared by zone 166(1) and zone 166(2), neither zone is available to detect subsequent scintillations until the PMT signals correlating to the event have been processed. Thus, during processing of the event, zone 166(1) and zone 166(2) may not detect a coincidence event.
Another disadvantage results from the prior art techniques used to overcome the non-uniform images generated by PET systems. In particular, prior art PET systems use calibration techniques to overcome the generation of non-uniform images. Typically, the calibration technique involves the characterization of the sensitivity profiles of the coincidence detection system. Subsequently, the characterization profiles are used to reduce the non-uniformity of the PET generated image. The use of calibration techniques, however, tends to result in the loss of image data.
An apparatus and method for an event detector system are described. The event detector system comprises a detector having a plurality of zones, each zone comprising a plurality of detector devices. For one embodiment, each detector device is operable to generate an event signal in response to an energy source. The event detector system also comprises a threshold circuit coupled to a detector device of the detector. For another embodiment, the threshold circuit is operable to selectively transfer the event signal of the detector device. Additionally, the event detector system comprises a summation circuit coupled to the threshold circuit. For yet another embodiment, the summation circuit is operable to generate a trigger signal in response to the event signal of the detector device.
The method for selecting a threshold value in an event detector comprises determining a detector configuration. The method also comprises applying a first threshold value to a threshold circuit. Additionally, the method comprises generating an energy source. For one embodiment, generating the energy source comprises directing a light source on the event detector. The method further comprises examining the detector configuration to determine the number of triggering channels engaged by the energy source.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.